(CNN) -- There are more than 200 men behind bars in the U.S. prison at Guantanamo Bay, Cuba. In a way, U.S. President Barack Obama is trapped there too.

Obama has pledged to shut down Guantanamo Bay but achieving that goal is proving harder than he hoped.

"How we deal with those situations," he said in an interview with the Associated Press earlier this month, "is going to be one of the biggest challenges of my administration."

The detention center for suspected terrorists is one of the most infamous prisons in the world. On his first full day as president, Obama promised to shut it within a year.

On Tuesday he'll be at the half-way point and he's still struggling to close it.

The president says there are three kinds of prisoners at Gitmo. First, there are those who'll be put on trial. Military courts have already heard some cases and civilian court proceedings are under way as well.

There are prisoners who are expected to be released or transferred somewhere else. Since Gitmo was established in 2002, hundreds have been sent to their home countries or other places willing to accept them.

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Finally, there are those the U.S. can't prosecute but doesn't want to let go.

All the cases are complicated but that third category is the most controversial: prisoners whom the administration still considers a threat, but doesn't expect to be able to convict in court using evidence judges will accept.